useScrollToBottom (user-message-anchor) undershoots on thread load — lands several messages before the last user message

[rabisg]

Bug Report: useScrollToBottom with user-message-anchor doesn't reach the last user message on thread load

Package versions

• @openuidev/react-ui: 0.9.20
• @openuidev/react-headless: 0.7.11
• React: 19.2.4

Describe the bug

When loading a thread with many messages (via loadThread), the scroll
position lands several messages before the last user message instead of
placing it at the top of the viewport. The user must manually scroll down
to reach the end of the conversation.
The desired behavior — last user message pinned to the top of the
viewport — is correct; it's just not being achieved.

AI Analysis

The user-message-anchor branch in useScrollToBottom (lines 76-89)
does the right thing in principle:

const lastUserMessageDiv = Array.from(
  element.querySelectorAll(userMessageSelector)
).pop();
const scrollPosition =
  lastUserMessageDiv.getBoundingClientRect().top -
  element.getBoundingClientRect().top +
  element.scrollTop;
element.scrollTo({ top: scrollPosition, behavior: "smooth" });

The scroll fires in the useEffect on line 93-103 the moment isLoadingMessages flips to false. The issue is that at that point the message components haven't finished laying out — complex content like OpenUI components, Markdown blocks, tool-call accordions, and images are still mounting and expanding. As a result:

getBoundingClientRect() returns stale geometry — the positions are based on partially-rendered content, so the calculated scrollPosition is too low (too far up in the chat).

behavior: "smooth" locks in the stale target — the animated scroll commits to the position computed at call time. As content continues to expand below the measurement point the target becomes increasingly wrong, but smooth scroll doesn't re-evaluate.

Together these cause the scroll to land several messages short of the actual last user message.

Steps to reproduce

• Use Shell.ScrollArea with the default scrollVariant ("user-message-anchor").
• Have a thread with ~20+ messages, ideally containing assistant responses with rich content (OpenUI Renderer output, Markdown with code blocks, tool calls).
• Load the thread (click it in the sidebar → loadThread).
  Observe: the viewport lands a few messages above the last user message. Scrolling down manually reveals the remaining messages.

Expected behavior

The last user message should be positioned at the top of the viewport after the thread finishes loading.

[abhithesys]

The issue is that at that point the message components haven't finished laying out — complex content like OpenUI components, Markdown blocks, tool-call accordions, and images are still mounting and expanding.

AI evaluation is accurate. I think we can add a resize observer and scroll to bottom unless user has interacted with the message list.